As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased. Among such secondary batteries is a lithium secondary battery having high energy density and operating voltage and excellent preservation and service-life characteristics, which has been widely used as an energy source for various electronic products as well as for the mobile devices.
Also, the secondary batteries have attracted considerable attention as an energy source for electric vehicles, hybrid electric vehicles, and plug-in hybrid electric vehicles, which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuel. As a result, the secondary batteries are being applied to an increasing number of applications owing to advantages thereof, and, in the future, the secondary batteries are expected to be applied to even more applications and products.
As applications and products, to which the secondary batteries are applicable, are increased, kinds of batteries are also increased such that the batteries can provide powers and capacities corresponding to the various applications and products. Furthermore, there is a strong need to reduce the size and weight of the batteries applied to the corresponding applications and products.
For example, small-sized mobile devices, such as mobile phones, personal digital assistants (PDA), digital cameras, and laptop computers, use one or several small-sized, lightweight battery cells for each device according to the reduction in size and weight of the corresponding products. On the other hand, middle- or large-sized devices, such as electric bicycles, electric motorcycles, electric vehicles, and hybrid electric vehicles, use a middle- or large-sized battery module (or battery pack) having a plurality of battery cells electrically connected with each other because high power and large capacity is necessary for the middle- or large-sized devices.
In a middle- or large-sized secondary battery module including a plurality of battery cells (unit cells), all of the battery cells or some of the battery cells are connected in series to one another so as to provide, particularly, high power. Preferably, the battery cells have the same capacity and voltage standard. Despite having the same standard, however, the battery cells generally have different voltages due to limitation in a manufacturing process caused by various factors. That is, there is a voltage difference between the respective battery cells. If the battery cells constituting the battery module have different voltages, the performance of the battery module is deteriorated. Also, a voltage difference between the respective battery cells may occur due to various factors during use of the battery module as well as during manufacture of the battery module.
Therefore, there is a high necessity for technology that is capable of minimizing a voltage difference between battery cells to ensure that a battery module operates in an optimal state.